CHAPTER 2: MOLES, DENSITY, AND CONCENTRATION

Transcription

1 CHAPTER 2: MOLES, DENSITY, AND CONCENTRATION 2.1 The Mole mole is a certain amount of material corresponding to a specified number of molecules, atoms, electrons, or any other specified types of particles. In SI system a mole is the amount of a substance that contains as many elementary entities (6.022 x )as there are atoms in kg of carbon 12, or there are x atoms of carbon in 12 grams of carbon-12 To keep the units straight we will use the gmole for the SI mole. The pound mole (Ib mol, comprised of x x 453.6) molecules, the kg mol (kilomole, kmol, comprised of 1,000 moles), and so on.

2 MOLES, DENSITY, AND CONCENTRATION molecular weight (MW) = mass moles the g mol = the lb mol = mass in g molecular weight mass in lb Molecular weight Example:

3 the atomic weight of an element is the mass of an atom based on the scale that assigns a mass of exactly 12 to the carbon isotope. the molecular weight of the compound is nothing more than the sum of the weights of atoms of which it is composed

4 Example: Use the Molecular Weights to Convert Mass to Moles If a bucket holds 2.00 Ib of NaOH, how many: (a) Pound moles of NaOH does it contain? (b) Gram moles of NaOH does it contain? Solution:

5 Example: How many pounds of NaOH are in 7.50 g mol of NaOH? Solution: Basis: 7.50 g mol of NaOH

6 2.2 Density Density is the ratio of mass per unit volume, as for example, kg/m 3 or lb/ft 3. Example: the density of n-propyl alcohol is g/cm 3, what would be the volume of 90.0 g of the alcohol? Solution:

7 molar density (ρ/mw) molar volume (MW/ρ) For some solutions, to calculate the density of the solution, you can make a linear combination of the individual components by adding the respective masses and volumes, and then dividing:

8 2.3 Specific Gravity Specific gravity is a dimensionless ratio. Actually, it should be considered as the ratio of two densities-that of the substance of interest, A, to that of a reference substance. The reference substance for liquids and solids normally is water which has a density of g/cm 3, 1000 kg/m 3, or lb/ft 3 at 4 O C.

9 The specific gravity of gases frequently is referred to air, but may be referred to other gases. Example: If a 70% (by weight) solution of glycerol has a specific gravity of at 15 o C, what is the density of the solution in: (a) g/cm3? (b) lb m /ft 3? and (c) kg/m 3? Solution: assume that the temperatures of the water is 4 o C, and that water has a density of 1.00 x l0 3 kg/m 3 (1.00 g/cm 3 ). (a) g solution/cm 3 (b) (1.184 lb glycerol/ft 3 )/(1 Ib water/ft 3 ) x (62.4 Ib water/ft 3 ) = 73.9 lb solution/ ft 3 (c) x l0 3 kg solution/m 3.

10 In the petroleum industry the specific gravity of petroleum products is often reported in terms of a hydrometer scale called o API. The equation for the API scale is:

11 Example: In the production of a drug having a molecular weight of 192, the exit stream from the reactor flows at a rate of 10.5 L/min. The drug concentration is 41.2% (in water), and the specific gravity of the solution is Calculate the concentration of the drug (in kg/l) in the exit stream, and the flow rate of the drug in kg mol/min. Solution: Basis: kg solution

12 Next convert the amount of drugs of solution to mass per volume To get the flow rate, take a different basis, namely 1 minutes

13 2.4 Flow Rate the flow rate of a process stream is the rate at which material is transported through a pipe Mass flow rate = Volumetric flow rate = Molar flow rate =

14 2.5 Mole Fraction and Mass (Weight) Fraction Mole fraction is the number of moles of a particular compound in a mixture or solution divided by the total number of moles in the mixture or solution. the mass (weight) fraction is the mass (weight) of the compound divided by the total mass (weight) of all of the compounds in the mixture or solution.

15 Example: An industrial-strength drain cleaner contains 5.00 kg of water and 5.00 kg of NaOH. What are the mass (weight) fractions and mole fractions of each component in the drain cleaner container? Solution: Basis: 10.0 kg of total solution

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17 2.6 Analyses of Multi-component Solutions and Mixtures The composition of gases will always be presumed to be given in mole percent or fraction unless specifically stated otherwise. The composition of liquids and solids will be given by mass (weight) percent or fraction unless otherwise specifically stated, as is the common practice in industry.

18 Example: what is the molecular weight of air based on the assumption that all of the air that is not O 2 is N 2 with a pseudo-molecular weight of Solution:

19 2.7 Concentration Concentration generally refers to the quantity of some substance per unit volume Mass per unit volume (lb of solute/ft3 of solution, g of solute/l, lb of solute/barrel, kg of solute/m 3 ). Moles per unit volume (lb mol of solute/ft 3 of solution, g mol of solute/l, g mol of solute/cm 3 ). Parts per million (ppm); parts per billion (ppb), a method of expressing the concentration of extremely dilute solutions.

20 Parts per million by volume (ppmv) and parts per billion by volume (ppbv) Other methods of expressing concentration are molarity (g mol/l), molality (mole solute/kg solvent), and normality (equivalents/l).

21 Example: A solution of HNO 3 in water has a specific gravity of 1.10 at 25 C. The concentration of the HNO 3 is 15 g/l of solution. What is the Solution: a. Mole fraction of HNO 3 in the solution? b. Ppm of HNO 3 in the solution?

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